This invention relates to a molding machine and more particularly to a machine for continuously molding articles from a strip of plastic compound such as rubber.
This invention is an improvement over machines of the type disclosed in U.S. Pat. Nos. 3,079,633 and 3,280,431. These machines include a large rotatable wheel that carries fixed and axially movable dies uniformly spaced about its periphery. At a molding station, a strip of uncured material is fed between the dies which then close upon the strip to bite off a piece from the strip and mold it to the desired shape. The wheel is heated and in turn the dies are heated from the heat in the wheels to cure the molded articles during the rotation of the wheel through the major part of a revolution.
At the end of the excursion of the part through the curing process, the dies are opened, and the cured part is ejected. It is known to apply a blast of air to the dies as the part is ejected to assist in the ejection and to clear the die of unused compound. The air is applied through a passage that extends from the die to the peripheral surface of the wheel. Compressed air is delivered to that passage by a nozzle past which the peripheral surface slides as the wheel rotates.
Although this type of molding machine has proved to be successful, the occurrence of trapped air within the mold cavity during closing of the dies has resulted in detrimental effects on part quality. Air pockets can cause visual or functional defects in the molded article because the uncured material will set and cure to a permanent shape within the mold cavity shortly after the dies close. Trapped air within the closed mold cavity can force the uncured material to take a shape other than that defined by the inside dimensions of the mold cavity.
Occasionally, the mold cavities are "burped" of this excess air by providing for a quick opening and closing of the closed dies shortly after the dies have closed on the molding compound. This practice has not completely solved the problem of trapped air. Further, the reopening and closing of the dies, once closed on the molding compound, may cause a distortion of the part.
The occurrence of visual and functional defects due to trapped air requires intensive inspection of the finished molded articles. A percentage of these finished molded articles may be rejected and scrapped due to these visual or functional defects.
The shape of the articles molded in these continuous molding machines has also been limited to rather shallow die structure. Because the dies close in an axial direction, the dimension of any molded article in the axial direction is necessarily limited to the distance that the uncured material will flow axially to fill the mold. This maximum axial dimension of molded articles is currently about 11/4".
The thickness of the strip of uncured material from which the articles are molded generally ranges from 1/4 inch to 1/2 inch and therefore some flow in an axial direction is required to fill the mold cavity for all molded articles having an axial dimension greater than 1/2". The greater the depth of the mold cavity, the greater the axial flow required thereby increasing the occurrence of trapped air. This increase in trapped air leads to an undesired increase in visual and functional defects.
It is an object of this invention to provide for mass production of molded articles with a reduced rate of rejected parts, thereby alleviating the need for intensive inspection of finished molded articles after the molding process.
It is another object of this invention to provide continuous molding apparatus that permits an increase in the axial dimension of the molded article.
It is yet another object of this invention to provide an apparatus for mass production of molded articles which does not require "burping" of the mold cavity to release trapped air.